1. Field of the Invention
This invention relates to printers which place a series of dots on underlying media to form a pattern, alpha numeric symbols, or a bar code. It relates more to those types of printers which are thermal printers wherein a print ribbon having a wax or other displaceable material thereon can be heated and disposed on an underlying media for printing thereon. Such underlying media can comprise paper, plastic, a web supporting a plurality of labels, or other media. The invention specifically relates to the print ribbon being driven and matched to the underlying media in a consistent manner to avoid various printing inconsistencies. Such printing inconsistencies can be light or dark print, improper alpha numeric symbols, or fuzzy printing as well as bar codes having either unclear or improper separations.
2. Description of the Prior Art
The prior art of thermal printers relied upon various brakes, clutches and other apparatus in order to provide for the proper control of the print ribbon. The print ribbon has material thereon such as a wax or other type of heat sensitive material which can be used to imprint underlying media. The print ribbon has a very flexible and thin consistency. It borders on the fineness of a film like material of a flexible plastic sheet.
Disposed on the print ribbon is the print substance which must be disposed on underlying media. The substance of the print ribbon which is disposed under heated conditions is placed on the underlying media. It is placed at discrete points that must be accurately maintained. The accuracy is with regard to alpha numeric representations and particularly with regard to bar codes which have to be properly read.
During the process of displacement of the substance from the print ribbon, a heating element is used. The heating element can be an elongated bar having very discrete heating elements that conform to a certain number of dots per inch as desired. Such dots per inch in the way of heating elements can range upwards of 600 dots per inch and more.
The print ribbon when passing under the heating element and on top of an underlying media is subject to wrinkling, striations, displacement, stretching, and other distortions. This is caused by tension, inertia, and other elements in the drive systems. In the past, it has been customary to compensate for these distortions with various clutches, controls, and mechanical elements which although workable in some cases do not always provide the best results.
Further complicating this matter is the fact that the underlying media that is to be printed on must be driven over a platen which is a rotatable platen formed of a hardened elastomeric material against which the print ribbon is guided and heated by the heating elements of the print head. Oftentimes, the print ribbons become mis-matched with the underlying media, and distortions occur which can be quite severe in a bar code are encountered.
In order to overcome the foregoing deficiencies, this invention utilizes a positive drive system of the print ribbon with respect to the underlying media. This is accomplished by means of a pair of D.C. motors that drive the take-up and supply spools.
The motors are driven by a current control drive circuit which is controlled by rotation of the motors. The motor velocities are measured by circuits that measure the Back EMF (BEMF) voltage of the motor drives. The tension on the print ribbon can then be a function of the spool radius and the motor torque, as well as inertia and other dynamic aspects including the mass of the rolls on both the take-up and supply spools.
The media is driven by a stepper motor connected to a stepper motor drive through an electrical circuit that applies power to the stepper motor.
An important feature of this invention over the prior art is the fact that it maintains the media stepper motor driver in a consistent manner with the passage of the print ribbon.
Another object of this invention is the control of the tension on the print ribbon. It is particularly important as it passes through the print head and over the underlying media that is to be printed. This particular control of the print ribbon is enhanced by the measurement of the velocity and other aspects of the print ribbon feed motor and take-up motor respectively.
The foregoing control is effected by BEMF sensors that are connected to the respective D.C. motors for the take-up and the drive spindles on which the ribbon spools are connected.
The BEMF sensors of the respective D.C. motors do not require any additional cabling. Fundamentally, the BEMF is measured across the motor windings. Since there is a cable that already runs from the motor windings to the controller, this cable is used for sensing as well as driving the motor.
Other aspects of the prior art have utilized various sensors to mount several of them at various locations to sense the movement of the print ribbon, and then run independent cabling to them to achieve the same result as using the BEMF sensors of the motors.
A further enhancement is that the ribbon tension can be varied with differently sized ribbon width. The prior art as previously stated to the contrary uses mechanical tension devices to maintain the tension on the print ribbon. Such mechanical tension devices can't vary the ribbon tension with respect to the ribbon width or the spool diameter. An object of this invention is to avoid the foregoing deficiencies by varying the tension to accommodate such changes thereby substantially lessening print ribbon wrinkle.
Another object of this invention is to provide a low or out ribbon indicator which can be added without having to add any other sensors. Fundamentally, since the method used to control the ribbon requires calculating the supply spool radius, the detection of the ribbon being low or out can be added without the need for additional hardware.
A full take-up spool indicator is also utilized without having to add additional sensors. This is based upon the fact that the method used to control the ribbon requires calculating the take-up spool radius. Thereby without adding additional hardware, determination of the take-up spool being filled can be accomplished.
Another object of this invention is that it provides for tensioning with a backing up tension device. When mechanical devices are used to maintain tension, especially friction type devices, another mechanism needs to be added to maintain the tension. This is usually a spring wrapped around a cam. This invention removes the need for this additional mechanism.
A further object of this invention is that when the media is jammed, there need not be any additional detectors. This is based upon the fact that the reading with respect to the overall movement of the media is detected by the invention so that a jam can be easily sensed and detected.